This makes the <mo> operators use the lspace/rspace properties to
determine their spacing. The spacing is now consistent with the one
of <mfenced> and as a consequence bug 118843 is fixed. This also ensures
that the rendering of <mo> elements is updated when the form or
attributes are changed.

- // FIXME: this should be removed when operator spacing is implemented (https://bugs.webkit.org/show_bug.cgi?id=115787). At the moment spacing for normal <mo> elements is handled in mathml.css and mfenced uses the arbitrary constants below.